Vulcanized fiber



Patented Sept. 24, 1935 VULCANIZED FIBER James H. Young, Arden, Del., assignor to E. I. du Pont de Nemours & Company, Wilmington, Del a corporation of Delaware No Drawing. Application November 13, 1934, Serial No. 752,888

Claims.

This invention relates to vulcanized fiber, and more particularly to the manufacture 'of vulcanized fiber from chemical wood pulp.

In the manufacture of vulcanized fiber it has 5 been the practice to treat with zincv chloride or other gelatinizing agents, paper made from bleached rag pulp of a high alpha cellulose content and a low viscosity in cuprammonium hydroxide. The dry sheets of paper are passed through a bath of zinc chloride or a mixture of zinc chloride with other'chemicals to cause par-..

tial gelatinization of. the cellulose fibers. The

sheets are laminated to give the desired thickness, cured for a short time in air 'atroom temperature, and then subjected to pressure and heat to insure good adhesion of the'individual plies. 'The zinc chloride is gradually removed from the sheet by a systemof counter-current washing. The final steps of the process are drying and pressing or'rolling. A less dense product is made .by using a rag pulp which will react slowly with the zinc chloride or by using sulfite wood pulp of high alpha cellulose content and low viscosity. In'all instances it is essential for the success of this prior art method that the pulp used be one of low viscosity and high alpha cellulose content.

Attempts have also been made to use the various chemical wood pulps as a base material for vulcanized fiber. These wood pulps have been prepared by special cooking methods and have been-given special treatments in order to-produce a pulp corresponding in chemical properties to that of a satisfactory rag pulp, namely, a pulp of low viscosity and of high alpha cellulose content. The cost of preparing vulcanized fiber using these high alpha cellulose wood pulps is, however, much greater than it is for processes utilizing standard grades of wood pulp of low alpha cellulose content.

This invention has as an object a process for utilizing the inexpensive low alpha cellulose chemical wood pulps in the manufacture of vulcanized fiber. A further object is an improved process for making vulcanized fiber. Other objects will appear hereinafter.-

These objects are accomplished by treating a chemical wood pulp of low alpha cellulose content but of high viscosity with mercerizing caustic, cutting the fibers to reduce their length, forming the fibers into sheets of paper, and vulcanizing the fibers with zinc chloride as will be more fully described hereinafter.

The caustic treatment to which the pulp in my process is initially subjected, is the superficial mercerization of the fibers with strong caustic as described in U. S. Patent 1,857,100. The pulp produced in accordance with this patent has, however, not found heretofore wholly successful use as a base'material for vulcanized fiber because of the lack of knowledge of certain essen- 5 tial requirements and of procedural steps which are disclosed herein. Thus, I have found thatthe mentioned caustic treatment is effective in the present process only if this treatment is appliedv to pulps which are specially selected because of 10 their high viscosity. I have also discovered that it 1 is essential to cut the fibers of the mercerized pulp to reduce the fiber length before forming the pulp into paper prior to the zinc chloride treatment.

The pulp which receives the above mentioned mercerizing treatment should have a viscosity in cuprammonium hydroxide of from 700 to about' 1200 centipoises. I prefer to use pulps having an alpha cellulose content below 88%. Such pulps are known as low alpha cellulose pulps, since 'pulps containing more than 93% alpha cellulose are referred to in the art as high alpha cellulose pulps. The alpha cellulose content rises somewhat after the caustic treatment but is still usually outside the range for high alpha cellulose pulp. The initial viscosity of the pulp I use changes but slightly and, hence, is distinguished sharply from the high alpha cellulose pulps of 90 to 200 centipoises viscosity in cuprammonium. For the purposes of the present invention, wood pulps which before the mercerizing treatment 'have a viscositydn cuprammonium below 700 centipoises are unsatisfactory because the vulcanized fiber made therefrom is quite brittle.

In addition to the requirements for viscosity mentioned above it is important that a bleached pulp be used in the first step of my process which consists in the mercerizing treatment described above. I have found that a mercerized bleached 40 chemical wood pulp of low alpha cellulose content and a high viscosity reacts with zinc chloride solutions in the same manner as a pulp prepared from cotton rags'of a high alpha cellulose content and a low viscosity. The vulcanized fiber prepared from my low alpha-high viscosity base has the same physical properties as vulcanized fiber prepared from the standard high alpha-low viscosity base made from cotton rag. Examples of suitable pulps are bleached. sulfite, bleached sulfate and bleached soda pulp. The best results are obtained with a bleached sulfite pulp.

The use of a bleached pulp is important not because of color but because of certain properties imparted by the/bleaching process to the pulp.

The bleaching reduces the viscosity of the cellulose and also removes 'lignin which prevents the zinc chloride from having the required gelatinizing action on the cellulose.

The cutting of the fibers after the mercerizing treatment is an essential step in the present process. It is important to observe, however, that this step of cutting the fibers into short, blunt sections must be sharplydistinguished from the conventional beating of the fibers with a large amount of water. In the latter instance although the fiber length is modified by the hydration and by the fraying of the fiber ends-through continual rubbing action, and although the freeness is considerably decreased the properties of the paper or felt made from the fibers is radically different from those of the felted cut fibers of the present invention. The sheet prepared from the beaten fibers is quite dense. The hydrated fibers are tightly cemented together and liquids are there-' fore absorbed but slowly. The sheet made from the cut crinkled fibers, on the other hand, will be weaker, of lower density and more absorbent. This sheet is much more absorbent than a sheet made from unmercerized fibers cut to the same freeness as the crinkled mercerized fibers used in the present invention. v The value of the step of cutting the fibers used in my process will be apparent from the resulting advantages mentioned below as compared to the disadvantages of using the uncut and unmercerized fibers. Cut unmercerized fibers could not of course be used at all because the sheet does not absorb enough zinc chloride to cause gelatinization. When the cut mercerized fibers are used there is markedly less shrinkage after removal of the zinc chloride and drying than when the sheet is made of long fibers. The directional shrinkage also is greatly reduced because of' the more heterogeneous deposition of the short cut fibers in the sheet. On the other hand, in the case of a sheet made from the long uncut fibers there is considerable directional shrinkage due .to the alignment of the greater part of the fibers inthe machine direction. Furthermore, cutting increases the area of the cellulose fibers that can be attacked by the zinc chloride solution and gelatinization is therefore more uniform.

The details for the superficial mercerization of the fibers of the pulp is fully set forth in the patent referred to above. The ordinary low alpha cellulose chemical wood pulp of /1 to 4 mm. fiber length, which for the present purpose must have the high viscosity described above, is mixed by efiicient dispersing apparatus at high pulp consistencies, namely, consistencies from 15 to 20%, with sodium hydroxide of mercerizing activity. For the present purpose sodium hydroxide solutions of 10 to 18% concentrations at room temperature are suitable.

The short time of the caustic treatment together with the small proportion of liquor to pulp prevents any drastic action of the caustic on the pulp and results in a superficially mercerized crinkled pulp of high freeness. This mercerizing treatment of the high viscosity-low alpha cellulose pulp is essential in the present process. First, the surface of the fibers is changed so that the zinc chloride solution does not react so quickly with the cellulose. Second, the curled condi tion of the pulp makes it possible to reduce materially the length of the fibers by cutting without substantially affecting the absorbency of the sheet prepared from the pulp. This is very important because the shrinkage that occurs when the sheet of fibers is treated with zinc chloride is very greatly reduced when the length of the fibers is shortened by cutting. If the pulp is not first mercerized and. if it reacts with zinc chloride, it

shrinks abnormally. Cutting the fibers of such 5 pulp lowers the absorbency of the sheet to such an extent that it does not impregnate thoroughly with the zinc chloride solution.

After the pulp has been freed from the mercerizing liquor, it is subjected to a cutting treatment in Hollander-type beaters or Jordans to reduce the fiber length. The fibers may be dyed at this stage if so desired. They may then be cut by a final Jordan treatment to reduce their length sufficiently to give good sheet formation with standard paper making equipment and to prevent excessive shrinkage when the sheet is subjected to the gelatinization treatment with zinc chloride solution. The mechanical treatment of the pulp should be such that the freeness is reduced from 20 about'25 to 55 seconds to about 160 to 250 seconds. The method of measuring freeness is described in the patent of reference.

The paper web, made by standard methods from the pulp treated as described above, is then passed through a solution of 70.5% zinc chloride at 50 C. The treated sheet may be rolled onto a mandrel to form tube stock or it may be used in a continuous process to produce thin sheets. Several sheets of paper may be treated simultaneously in the same bath and laminated into sheets by the application of pressure and heat, and the resulting multiple sheet is then washed and dried in a continuous operation. The treated tubes or sheets are washed in successively weaker solutions of zinc chloride. The final traces of zinc chloride are removed by washing the material in running water. The length of time required for curing the fiber depends on the thickness of the sheet. The washed fiber is finally dried and pressed.

The following examples illustrate more specifically the method for practicing my invention:

Example I A bleached sulfite chemical wood pulp of the following measurable properties:

was treated with an 18% caustic soda solution as 50 follows:

Pulp 20 arts 18% NaOH solution parts Temperature of NaOH solution 25 C.

55 The time of treatment is preferably not greatly in excess of that necessary to wet thoroughly all of the fibers with the caustic solution. With suitable equipment this is usually accomplished in less than a minute and even within the range 60 of 15 to 30 seconds. When wet pulp is used instead of the dry pulp in the present example, it is necessary as more fully pointed out in the mentioned patent that the caustic concentration be adjusted to compensate for the known amount of moisture in the pulp.

The pulp was intimately mixed with the caustic soda solution to produce a superficial gelatinization of each fiber and to loosen them from the sheet form. After being treated the pulp was promptly washed free of caustic soda with water. The Washed pulp was puffed due to the curling and cr'mkling of the fibers and on examination under the microscope it was noted that the individual fibers of the washed pulp had a crinkled 75' Viscosity in cuprammonium 858.0 centipoises Alpha cellulose content 90.50% Freeness 31 seconds The fiber length of the pulp was reduced by cutting in a Hollander-type beateruntil the freeness was 200 seconds.

Sheets .0070" thick were made from this pulp and weredried at '100-110 C. The dry paper was immersed 10 seconds in a 70.5% zinc chloride solution at 50 C. The treated sheets were rolled onto' diameter mandrels and cured for two hours in the air at room temperature. The tubes were then immersed in a 20% zinc chloride solution for.24 hours, removed from mandrels, and immersed in a 10% zinc chloride solution for '72 hours. The tubes were then immersedin water for '72 hours and finally washed with running water for two hours at which pointno precipitate formed when the tubes were immersed in a dilute solution of ammonium hydroxide. The tubes were placed on a mandrel and dried at 65 C. They were then rolled and polished under standard conditions of pressure and speed. No fracture resulted during the rolling test, and no blisters or untreated spots were evident. The physical properties of the finished vulcanized fiber follow:

Shrinkage 14.5%

Specific gravity 1.519

Hardnessv 44.2

Example II A bleached sulfite chemical wood pulp of the following properties: Viscosity in cuprammonium 1118.0 centipoises Alpha cellulose content 85.7%

Freeness seconds was treated with an 18% solution of caustic soda using the procedure cited in Example I. The properties of the treated pulp were as follows:

Viscosity in cuprammonium 962.0 centipoises Alpha cellulose content 93.2% Freeness 47 seconds as follows:

Shrinkage 9.4% Specific gravity 1.540 Hardness 49 Examp e III A bleached sulfite chemical wood pulp of the following properties:

Viscosity in cuprammonium 790.8 centipoises Alpha cellulose content 86.17% Freeness seconds was treated'with a 12% caustic "soda solution as follows: I

Pulp 20 parts" 12% NaOH solution 80 parts{ parts NaOH 70.4 parts H2O The same procedure was used for treating the pulp with caustic soda solution as was used for Example I. Properties of the treated pulp were as follows: I

Viscosity in" cuprammonium 914.0'centipoises' Alpha cellulose content 89.51% 5 Freeness 37 seconds The 'fiber length of the pulp wasreduced by cutting in asn'iall Jordan engine until the freeness was 225 seconds. The same procedure was 1o used for preparing the sheets and for the subsequent treatment with zinc chloride as was cited in Example I. The physical properties of the finished vulcanized fiber follow:

Shrinkage 12.0% Specific gravity 1.538 Hardness 48 Any of the agents useful for fiber vulcanization may be used for my process. Other suitable gel- 20 -atinizing agents are zinc chloride, calcium thiabove examples of viscosity and of reduction of fiber length after the mercerizing treatment be adhered to. For instance, in repeating the procedure of Example I, I reduced the fiber length so that the freeness was changed from 31 seconds to 60 seconds instead of to 200 seconds. The sheets had irregular distribution of the fibers (poor formation) and they were very absorbent. 35 They were treated with zinc chloride solution and made into tubes according to the procedure outlined in Example I.

The finished fiber contained blisters and the shrinkage resulting from the action of zinc chlo- 40 ride on the fibers was unusually great. The vulcanized fiber had the following characteristics, shrinkage 30.0%, specific gravity 1.500, and hardness 34.6.

The importance of the viscosity requirement is 45 shown by the following: A bleached chemical wood pulp of 85.7% alpha cellulose content but with a viscosity of only 652.0 centipoises was given the caustic treatment described in Example I. The treated pulp had a viscosity in cuprammo- 50 nium of 640.0 centipoises, an alpha cellulose content of 94.4% and a freeness of 22.6 seconds. The fiber length was reduced by cuttingto a freeness of 200 secondsv as before and the vulcanization with'the zinc chloride was likewise conducted 55 as in Example I. The tubes were fractured during rolling and polishing. Also the shrinkage resulting from the treatment with zinc chloride was 31.0%. Hardness and density tests could not be made because the tubes were too brittle to 00" make satisfactory vulcanized fiber.

The method used for measuring the viscosity of cellulose in cuprammonium hydroxide solution is the Standard A. C. 8., method described in the Analytical Edition of Industrial and En- 05 gineering Chemistry, vol. 1, Number 1, pages 49-51. -Briefiy, the determination consists-.01 measuring the time in seconds required for a inch diameter glass sphere to fall 15 centimeters through a 2.5% solution of cellulose in 70 cuprammonium hydroxide that contains 30 g. of copper,- gJof ammonia, and 10 g. of sucrose per liter (1000 cubic centimeters). The viscosity is generally expressed as centipoises which is calculated by means of a standard formula. 75.

involving the .factors of density of the .glass sphere, density of the solution, time required for sphere to fall, and a known conversion con- I stant. It is to be understood that the viscosity 5 figures used in the claims refer to a viscosity determination made with cuprammonium in this manner.

= Alpho cellulose is defined as that form of cellulose which'is insoluble in an 18% caustic soda 1 solution at 20 C. when the treatment of the pulp with the caustic soda is carried out by the standard procedure.

The hardness figures given in the examples are the results of tests given with a Rockwell hardness tester as follows: A 9.5 millimeter diameter steel ball under a load of 100 kilograms was pressed against the sample of vulcanized fiber for a period of 15 seconds atroom temperature. The Rockwell hardness number recorded on the ;B scale of the instrument at the end of 15 seconds was the hardness value. A hard material shows a high reading and a soft material shows a low reading.

, The details in my invention are subject to various modifications as will be understood by those skilled in the art. Instead of mercerizing bleached chemical wood pulps, unbleached chemical wood pulps may be mercerized as outlined aboveand subsequently bleached after most of the mercerizing liquor has been removed from the pulp. It is necessary that the viscosity in ouprammonium and alpha-cellulose content of the bleached pulp fall within the limits which I have established. For preparing very tough vulcanized fiber that is not to be used for electrical insulation, I may blend unreactive fibers with the pulp prepared by my preferred process. The mercerized fibers will insure good absorbency in the sheet.

40 Any of the standard chemical pulping processes yielding a fully liberated wood pulp fiber may be used for preparing the pulp providing the specified viscosity in ouprammonium and alpha cellulose content are obtained, and the mercerized fibers are cut sufiiciently to give a sheet that will not shrink unduly in the process.

My invention is useful for manufacturing a paper base for vulcanized fiber that is used in electrical insulating work. This baseis characterized by uniformity, good absorbency, and the correct reactivity with zinc chloride solutions. These factors are of great importance in manufacturing any type of vulcanized fiber.

A particular advantage of this invention is that {certain standard chemical wood pulps which are relatively cheap, as compared *to rag pulp and high alpha cellulose wood pulps, can be made suitable forvulcanized fiber base by mercerizing the fibers. Chemical wood pulps have an ad- 69 vantage over rag pulp because their chemical properties can be regulated and controlled within narrow limits by the time, pressure and con+ centration of chemicals used during the cooking process. Also, a uniform source of wood is available, whereas rags represent a very heterogeneous collection that vary widely in chemical properties. My invention represents a further advantage to the manufacturer of vulcanized fiber in that he may purchase a pulp which is ready for conversion into paper. Another feature is that chemical wood pulps are not contaminated with solid metallic particles such as are found in cot- 10 ton rags. These metallic particles are very detrimental to electrical insulation. Although elaborate systems of riiiie boxes and screens are used to remove them, they are never completely removed so the final product has many weak spots. 15

As many apparently widely different embodiments of this invention may be made without de-' parting from the spirit and scope thereof, it is to be understood that I do not limit myself to the specific embodiment thereof except as de- 20 fined in the appended claims.

I claim:

1. A process for making vulcanized fiber which comprises treating'chemical wood pulp having a viscosity of about 700 to about 1200 centipoises 25 with a mercerizing agent under conditions which produce a mercerized fiber having not less than 25 seconds and not more than 55 seconds freeness, cutting the fibers to reduce the freeness within the range of about to about 250 sec- 30 .onds, forming the cut fibers into paper, and treating the paper with a gelatinizing agent.

2. A process for making vulcanized fiber which comprises treating bleached chemical wood pulps containing less than 88% alpha cellulose and hav- 35 ing a viscosity of about 700 to about 1200 centipoises with sodium hydroxide of mercerizing activity to produce a pulp having a freeness of from about 25 seconds to 55 seconds, cutting the fibers to reduce the freeness within the range of about 5 160 seconds to about 250 seconds, forming the cut fibers into paper and treating the paper with a gelatinizing agent.

3. In a process for making vulcanized fiber, the steps which comprise reducing the fiber length of crinkled mercerized wood pulp having a viscosity of about 700 to about 1200 centipoises, sufiiciently to cause good sheet formation and prevent excessive shrinkage when the sheet is subjected to gelatinization treatment for vul- 5 canization of the fiber, felting the cut fibers into i a sheet, and treating the sheeted fibers with a gelatinizing agent.

4. A process for making vulcanized fiber which comprises cutting mercerized bleached wood pulp 53 of high viscosity and high freeness to a freeness substantially lower, felting the cut pulp into paper, and treating the paper with a gelatinizing agent.

5. Vulcanized fiber substantially identical with that obtained'in accordance with the process set so forth in claim 1.

JAMES H. YOUNG. 

